As is known in the prior art, systems for controlling the cross-direction grammage of paper manufactured by paper machines operate in the following manner. The flow of thickstock coming into a wire pit of a paper machine is regulated by a grammage valve based on grammage measurement at the dry end of the paper machine. The grammage of the paper web is measured by means of measuring sensors traversing in a cross direction thereof, and the measurement result of the cross-direction grammage profile is averaged and passed as a feedback signal of a control system. The flow of thickstock is passed from the grammage valve, as known in prior art, to the wire pit, into which white water from the wire section of the paper machine is also passed. The thickstock flow and white-water are mixed in the wire pit and the thus-obtained diluted stock flow is passed, as known in itself, through pulp cleaning and deaeration devices to an inlet header of a headbox and therefrom, further through a distribution tube bank of the headbox, possibly through an equalization chamber and a turbulence generator to a slice channel of the headbox. A pulp suspension jet is discharged from the slice channel onto a forming wire or into a forming gap defined between forming wires.
In the prior art, the cross-direction grammage profile of paper manufactured by paper machines may be regulated by profiling the height of a slice opening in a headbox based on the aforesaid grammage measurement at the dry end of the paper machine. Recently, so-called dilution regulations have also become more common in which dilution water, usually white-water or a stock that is more dilute than the headbox stock, is supplied to individual feed points situated in the cross-direction in connection with a headbox. This dilution water feed system serves to profile the cross-direction grammage profile of a slice jet together with the regulation of a profile bar, or without it. A special advantage of dilution regulation is that the headbox can be operated with a slice opening having a uniform height so that the cross-direction flows in the slice jet and after it, caused by the profiling of the height of the slice opening, as well as distortions of the fiber orientation profile of paper resulting from them, may be avoided.
Prior-art dilution headboxes are subdivided into several feed zones across the headbox. Additionally, there may be more than one layer of such feed zones. Two or more streams of feedstuff supply each layer of feed zones, and each stream of feed stuff may supply one or more layers of feed zones. Each feed zone in each layer is equipped with means for controlling the combinatory proportions of streams fed to that feed zone, normally using a suitable valve arrangement. Additionally, there may be one or more layers of feed zones which are supplied by only one feed stream, or are supplied by plural feed streams without means for controlling combinatory proportions of feed streams fed to each feed zone.
Commonly, two feed streams are provided, one supplying the main feedstuff, and the other supplying a feedstuff of different properties. Normally, the second stream is more dilute than the main stream, but this need not always be so. The dilute feedstuff is normally white-water taken from the wire pit or short circulation, often with some processing, such as deaeration, cleaning, or filtration. The main feedstuff also normally contains white-water, to which a thickstock is added. In some cases, clarified water may be used instead of white-water as the dilute feedstock.
More than two feed streams may be provided, where each feed stream supplies feedstuffs of different material composition. For instance, both white-water and clear water streams may be supplied as well as the main feedstuff stream. Alternatively, two main feed streams may carry different feedstuffs, with a third feed stream carrying a dilute feedstuff However, it is also possible that more than one stream contains the same feedstuff; in this case, all streams carrying an identical feedstuff are treated as a single consolidated stream for the purposes of the invention described below. Applicability of this invention requires that not all streams cany exactly the same feedstuff, as described more fully below.
The streams fed to each feed zone are mixed together in any of several ways in the feed zone, producing an aggregate stream. The aggregate streams from all feed zones are merged, forming a single jet discharged across the whole headbox. There may be some mixing between streams in adjacent feed zones in this merging.
An example of a dilution headbox is the Valmet Sym-Flo D.TM..
With respect to different details of structures of dilution headboxes, reference is made to the following patents and patent applications: Finnish Patent No. 92229 (corresponding to European Patent Application No. 0 633 352 and U.S. Pat. No. 5,674,363) and U.S. Pat. No. 5,560,807.
In some cases, multiple conventional (non-dilution) headboxes may be operated as if their combination formed a dilution headbox. This is possible if the headboxes do not all have the same feed streams, and there is a difference in composition between some of the feed streams being fed to the individual headboxes. In this case, modulating the slice lip profiles has the effect of changing the combinatory proportions of the feed streams at each location across the web. However, the streams are not mixed, so the effect is similar to operation of a multilayer dilution headbox.
Feedstuffs, White-water, Retention
It should be noted that the feedstuffs used in the paper industry are of complex composition, containing many distinct material components suspended in an aqueous solution The principal material components are fibers of different kinds, with properties which depend on the fiber source (Norway spruce, silver birch, Eastern hemlock, bagasse, kenaf, etc.), and pulping process used. Resins and synthetic polymers, as well as various clays, minerals (ash), and other inorganic material may be added. Substances such as dyes, brighteners, anti-brighteners, bleaches, and opacity agents may occur in quantities which have negligible effects on the weight, strength, or other material properties of the web, but which have major effects on color, brightness, opacity, and other optical properties of the web. There may also be solutes dissolved in the aqueous solution, affecting its pH and other chemical properties, thus modulating the effect of other feedstuff components on properties of the web.
As known in the prior art, when initially forming a paper web, the aqueous solution is drained through porous fabric (the "wire" of a forming section) into the wire pit, as white-water, leaving much of the suspended material to substantially form the web. The white-water at each section of the wire contains substantially the same suspended components as the jet above it, but in lesser concentrations. Normally, white-water is combined from all sections of the wire into a single stream. When several forming units are used, as in manufacture of a multilayer web, the white-water streams from each forming unit may be kept separate in the process, or may be merged into a composite white-water stream.
The fraction of each component suspended in the jet which remains in the web is referred to as the "retention" of that component. Different components can have greatly differing retentions, and the retentions of some components is affected by chemical properties of the aqueous suspension (such as pH), and by concentrations of other components (such as polymers). Thus, the white-water varies in its component concentrations differently to the jet. Moreover, the retention of each component can vary differently with process conditions.
The retention of each component generally increases if the web is made heavier, but to different extents. Since properties such as weight may vary across the web, and since the composition of the jet can vary across the web, the retention of each component in the jet can also vary across the web. As the white-water from the wire pit is a mixture of white-water drained from all locations across the machine, only the average retention can be inferred from concentration measurements in the feed streams to a headbox.
Some paper machines make only a few grades of paper, and employ substantially the same feedstuffs under substantially similar process conditions whenever a particular grade is being manufactured. Under these circumstances, each grade likely has a characteristic narrow range of retentions, and there is little variation in concentrations of the main feedstock or the white-water.
More commordy, paper machines make a variety of grades from feedstuffs of diverse properties, and adjust process conditions accordingly. Under these circumstances, retention of each component can vary greatly within a single grade, and across grades. Similarly, white-water concentrations can vary differently for each component, both within and across grades. Large variations can occur over short times within a single grade.
Recycled fiber tends to be more variable in properties than new fiber, and its use is increasing in many paper machines. Use of a paper machine's repulped off-specification production (broke) varies from time to time, even in single grade machines.
Thus, the plural feed streams to a dilution headbox normally contain different concentrations of each feedstuff component. In general, the ratio of concentrations of a component in the several feed streams is different for each component. In particular, a white-water feed stream will be relatively richer in solutes and fine suspended solids than in fibers, and relatively richer in short fibers than in long fibers, when compared to the main feed stream.
Feedstuff Property Measurements
The physical and chemical properties of the major feedstuff components exhibit considerable variation. This is partly due to their natural origin, and partly due to variations in processing. These component variations, together with variation in blending of components to form a feedstuff, cause variation in the properties of feedstuffs. Variation in the operation of the short circulation of the paper machine can be a further cause of feedstuff property variation.
Until recently, it was laborious to perform more than a superficial laboratory analysis of concentrations and other properties of typical paper industry feed streams. Accordingly, paper mill laboratories measured only a total retention, and the practice in the paper industry is to treat retention as a single quantity. More sophisticated laboratory instruments are now available, but due to remoteness from the process and other practical concerns, analyses of headbox feed streams are infrequent. Moreover, a laboratory analysis is unlikely to be sufficiently timely for control purposes when retention is varying.
Devices which measure viscosity or freeness as an analogue of consistency (an aggregate concentration of suspended solids) have been available for many years, but have been of mediocre reliability and accuracy. The technology underlying such devices is also unsuited to low consistency regimes, such as those encountered in feed streams to the headbox. Accordingly, such devices have seldom been installed in headbox feed streams, and are not employed in cross machine control of dilution systems.
Newer, more sophisticated measurement devices are suitable for continuously and rapidly measuring concentrations of low consistency streams. These are capable of measuring distinct component concentrations, or distinct aggregate concentrations of groups of components (such as total ash concentration or total fiber concentration) as well as, or instead of measuring the total consistency.
An example of such a concentration measurement device is the device marketed by the trademark Kajaani RM-200.TM..
In addition to concentration, instruments are available for on-line measurement of other feed stream properties such as color and brightness of a sample, and for measuring the distribution of fiber lengths in a sample.
Other factors, such as pH or temperature, may determine the extent to which a feedstuff property affects web properties. Devices for measuring pH, various solvated ionic species (as pNa, pK, etc.), or temperature are commonly available, including some suitable for use in headbox feed streams.
Web Property Measurement and Control
Many properties of the moving web can be measured during manufacture of paper. Commonly, a paper machine is equipped with a number of measurement devices which traverse the moving web at one or more locations on the paper machine. Alternatively, an array of sensors may be deployed across the web, or stationary sensors may remotely measure properties across the web. Typical properties measured are basis weight, water weight, ash weight, caliper, gloss, brightness, opacity, fiber orientation, and strength. Some of these properties may be measured in greater detail, such as distinguishing between different species of ash (Al.sub.2 O.sub.3, CaCO.sub.3, SiO.sub.2, TiO.sub.2, etc.), or different resins. Other properties, such as dry weight, fiber weight, or percent moisture may be derived from these measurements.
These web property measurements are made in each of several subdivisions of the web in the cross machine direction, presented as a "profile" across the web. With modern measurement systems, the web subdivisions may be less than 1 cm in width. A control system for regulating the plural values of such a profile property commonly provides a means for entering the desired shape of the profile. Moreover, there may be several properties, each with a different desired profile shape.
Moreover, properties of the suspension discharged from the headbox may be measured during formation of the web on the wire. Such measurements should also be construed as web property measurements in the context of this invention, provided a property is measured at plural locations in the cross machine direction.
The ability to control the combinatory proportions of feed streams at each feed zone allows properties of the web to be controlled during manufacture. A change in combinatory proportions at all feed zones across the headbox can affect one or more properties of the web at all locations across the web. A change in combinatory proportions at a single feed zone can affect one or more properties of the web over a portion of the web. The width of the affected portion of the web may not correspond to the width of the feed zone, and the effect may be unevenly distributed in magnitude or sign within the affected portion of the web. When more than one property is affected, the effect on each property may be differently distributed over portions of the web which may differ in width and location.
The effect on a material property of the web, such as ash weight, of changing the combinatory proportion of feed streams depends on the different concentrations within those streams of each component which influences that property.
The effect on other properties of the web, such as color or opacity, depends both on material properties of the feed streams, and on non-material properties, such as brightness. The retention of each feed stream component over the affected portion of the web may also affect the magnitude of the effect, and this retention may be influenced by several measurable properties of the feed streams, such as pH or temperature.
A control system can more effectively modulate the combinatory proportions of the feed streams if it can more accurately model the process effect of such modulation on each of the properties to be regulated. Such modeling requires that the appropriate feed stream properties are measured, and that the dependencies between feed stream properties and web properties be substantially known. Many such dependencies are common knowledge.
Since the plural feed streams to a dilution headbox contain different relative amounts of the various feedstuff components, and since each feedstuff component affects one or more web properties to various extents, it is evident that changing the combinatory proportions of the feed streams can have tangible and dissimilar effects on plural web properties.
For example, if a dilution headbox utilizes two feed streams, one carrying white-water and the other carrying the main stock, the fiber in the web is supplied predominantly by the main stock stream, but the ash may be supplied in similar degree by both streams. Thus, changing the combinatory proportions of the feed streams at one or more feed zones will clearly affect the web fiber and ash profiles differently.
Nowadays, control systems exist which can effectively modulate one or more cross machine actuator systems to regulate one or more property profiles. The regulation of web properties can be enhanced by providing suitable measurements of properties of the plural feed streams to a control system, and utilizing process models which relate changes in web properties to the combinatory proportions of feed streams and to the properties of the feed streams.
An example of such a control system is the Valmet Damatic XD.TM..
Regarding prior-art control systems of a paper machine, reference is also made by way of example to U.S. Pat. No. 5,381,341 (corresponding to European Patent No. 0 401 188 and Finnish Laid-Open Publication No. 85731).
Introductory Summary
The salient points of the above discussion can be summarize as follows:
The plural feed streams to a dilution headbox contain different relative amounts of the various feedstuff components, and differ in other properties such as color, brightness, pH, temperature, etc. PA1 Each feedstuff component affects one or more web properties to various extents. Web properties are also affected by other properties of the feedstuffs, such as color, brightness, pH, temperature, etc. PA1 Changing the combinatory proportions of the feed streams can have tangible effects on plural web properties. The extent to which a web property is affected by a change in the combinatory proportions of feed streams depends on the properties of those feed streams. PA1 By deploying suitably accurate and reliable measurements of concentration or other pertinent properties of the feed streams to a dilution headbox, a control system can better regulate one or more property profiles of the web, by modulating the combinatory proportions of the feed streams at each feed zone of a dilution headbox. PA1 means for sampling continuously or at intervals each of two or more feed streams supplied to a headbox or headboxes; PA1 means for measuring properties of the feed streams, such as, concentrations of constituents in and/or brightness of and/or color of the samples; PA1 regulation means for regulating combinatory proportions of the feed streams at each of a plurality of feed zones of the headbox or headboxes in a cross direction thereof; and PA1 means for supplying the measurements of properties of the feed streams or factors calculated therefrom to the regulation means responsive to one or more measured properties of the paper web and modulating the aforesaid combinatory proportions to regulate the aforesaid properties of the paper web.
The last of these points leads to the present invention.